In the fifth major delay to its schedule for flying the 787 Dreamliner, Boeing said Tuesday it has discovered the plane needs added structural reinforcement where the wing and fuselage meet.

The news dealt the 787 program a serious new setback as the company's plan to reach first flight by June 30 went out the window.

When the hot-selling plane will fly is now uncertain: Boeing said it will be "several weeks" before executives even come up with a schedule.

In late May, Boeing engineers found a structural defect while conducting stress tests on the plane, the first to be built primarily out of carbon-fiber plastic composite.

After several weeks of analysis — and after the Paris Air Show gatherings of industry chieftains wound down — the company concluded late last week that, in the words of Pat Shanahan, head of airplane programs, "a productive flight-test program could not take place without structural reinforcement in limited areas."

Boeing Commercial Airplanes Chief Executive Scott Carson said "structural modifications like these are not uncommon in the development of new airplanes," and called the fix "quite manageable."

But investors reacted sharply, sending Boeing stock down $3.03, or 6.5 percent, to $43.87 at the market close. It was the stock's biggest drop in dollar and percent terms since Nov. 6, a day when the Dow index fell 4.8 percent.

Even before the latest delay, the 787 was nearly two years behind its initial schedule for takeoff.

Richard Aboulafia, an analyst with the Teal Group, said he's never seen an airplane rollout delayed this close to its first scheduled flight.

"Usually you get to this stage, and there are no showstoppers," he said. "That only increases the feeling that there are so many uncertainties here."

The news came just a week after Boeing executives at the Paris Air Show, including Carson, offered assurances that the plane would fly by month-end.

Carson said on the conference call that initial analysis suggested first flight could go ahead even as Boeing tried to fix the problem. However, late last week, after the executives had returned from Paris, the decision was made to halt first flight.

"While difficult, this was the prudent step for us to take," Carson said.

Scott Fancher, the head of the Dreamliner program, explained the structural problem in a conference call early Tuesday morning.

He said that late in May engineers, performing wing-bend tests on the airplane that is set aside inside the factory specifically for ground testing, found that strain gauges showed higher stress than predicted by the computer models at multiple points along the upper part of the wing-to-body join.

Fancher said the stress problem occurred at about 18 locations on each wing, in areas no bigger than 1 or 2 square inches each. The areas affected are where the wing made by Mitsubishi in Japan is joined to a part of the center fuselage body made by Fuji, also of Japan.

When engineers checked the areas flagged by the instruments measuring the strain, they found evidence that the structure was indeed stressed. Asked if that meant delamination — separation of layers in the composite material — Fancher didn't specify exactly but hinted that visible damage was apparent.

"We saw a number of things indicative of what the strain gauges were saying," Fancher said.

Boeing executives emphasized that the issue does not represent a fundamental problem with the new composite plastic material from which the airplane is constructed.

"Composites are the right material," said Shanahan. "We will correct this situation and do it with both care and urgency."

Boeing insists it can fix the problem with a modification to strengthen the structural areas in question. Fancher said that will involve "a relatively small number of parts and a relatively simple modification."

"We're talking about a couple of parts that can literally be held in your hand," at each of the 36 locations affected, Fancher said.

Fancher said that while the reinforcements will add "a small amount of weight, we expect negligible impact to airplane performance."

However, given the two years of delays leading up to this last-minute surprise, such assurances have a big credibility problem. The lack of a new schedule is also an issue.

Aboulafia speculated that the 787's service entry could be delayed until late 2010 or early 2011.

Boeing's original timeline for the Dreamliner called for first delivery in May 2008; until Tuesday it was saying the plane would debut with a delivery to Japanese airline All Nippon Airways (ANA) during the first quarter of 2010.

Boeing has amassed 866 orders for the Dreamliner through May. But this year, with the recession eroding airline traffic and making new orders difficult to finance, Boeing customers have only added 13 Dreamliner orders while dropping 58, according to the company's Web site.

The last-minute delay also further damages Boeing's credibility, said Aboulafia. "That's why that first flight was going to be a very important development. This reinforces the worst opinions of their doubters."

Regarding outsourcing, Aboulafia said it has worked well in other airplane programs but that there are legitimate questions about how much design work Boeing farmed out.

"It was already a technologically ambitious program," he said.

In the weeks ahead, Dreamliner chief Fancher said, engineers must assess several different possible modifications, select the best, then design and test it. In the meantime, he said, Dreamliner No. 1 will proceed as far as the taxi tests on the ground. But it won't fly.

Once a modification is developed, all the airplanes — those already built and those in process of being built — will have to be modified.

Customers have also begun to react. ANA issued a brief statement all but pleading for information on when it can get its airplane.

"We are disappointed that the first flight of the 787 will be postponed, and urge Boeing to specify the schedule for the program as a whole as quickly as possible," the ANA statement said.

Machinists union District 751 President Tom Wroblewski said he's glad Boeing found the problem. "That's what tests are about, is finding out the problems before you have a failure in flight."

He said he doesn't want to dwell on how many delays the 787 has experienced. "I'm hoping Boeing will have learned lessons from all the outsourcing that has gone on."

The financial impact of the delay will be disclosed when Boeing issues its second-quarter 2009 earnings report in July, the company said.